Spherical Colloidal Particles Research Articles

Fluid–triangular solid phase transitions in a system of two-dimensional nematic quadrupoles

ABSTRACTDensity functional theory of freezing is used to study the phase transitions in a system of spherical colloidal particles dispersed in nematic host confined to two dimensions. We have considered both the one-component and two-component systems of the colloidal dispersions. Particles are assumed to interact via director distortion-mediated purely repulsive potential which scales as the fifth power of the inverse interparticle separation. The pair correlation functions needed as input information in the density functional theory are calculated by solving Roger–Young integral equation theory. In one-component system, a triangular crystalline phase is found to be stable. On the other hand, considering the freezing of the fluid phase of the binary mixture into a substitutionally disordered triangular solid, the temperature–composition phase diagram is found to have spindle shape for the ratio of quadrupole moment of the particles of the components being 0.9 and 0.8. The phase diagram changes to an azeotrope at a ratio 0.7. The results are verifiable in real-space experiments on nematic quadrupoles confined to a two-dimensional plane.

An identification of the soft polyelectrolyte gel-like layer on silica colloids using atomic force and electron microscopy

A procedure is introduced for measuring the radius of spherical colloid particles from the curvature of upper parts of their central cross-sectional profiles obtained by atomic force microscopy (AFM). To minimize the possible compression and displacement of the spheres, AFM is operated in a mode rendering a constant ultralow pN force on the tip. The procedure allows us to evaluate the mean radius of nearly monodisperse submicrometer spheres of silica in their natively hydrated state in aqueous electrolyte solutions, irrespective of whether they are coagulated or not. A variation in the volume (swelling degree) of layers delimited by the AFM mean radii of these spheres in KCl solutions and their invariable mean radius in vacuum is obtained that follows a scaling power law derived in polymer physics for swellable polyelectrolyte gels and deduced previously by us from coagulation tests. This supports our former suggestion about the existence of soft polyelectrolyte gel-like layer developed spontaneously around silica surfaces and colloids. We discuss this finding in the context of recent knowledge about the structure of the silica/water interface obtained from direct surface force measurements between macroscopic silica surfaces and from particle size measurements of silica colloids and highlight its importance for colloid chemistry and condensed mattter physics.

Defects Around a Spherical Particle in Cholesteric Liquid Crystals

AbstractWe investigate the defect structures around a spherical colloidal particle in a cholesteric liquid crystal using spectral method, which is specially devised to cope with the inhomogeneity of the cholesteric at infinity. We pay particular attention to the cholesteric counterparts of nematic metastable configurations. When the spherical colloidal particle imposes strong homeotropic anchoring on its surface, besides the well-known twisted Saturn ring, we find another metastable defect configuration, which corresponds to the dipole in a nematic, without outside confinement. This configuration is energetically preferable to the twisted Saturn ring when the particle size is large compared to the nematic coherence length and small compared to the cholesteric pitch. When the colloidal particle imposes strong planar anchoring, we find the cholesteric twist can result in a split of the defect core on the particle surface similar to that found in a nematic liquid crystal by lowering temperature or increasing particle size.

Bio-Inspired Bright Structurally Colored Colloidal Amorphous Array Enhanced by Controlling Thickness and Black Background.

Inspired by Steller's jay, which displays angle-independent structural colors, angle-independent structurally colored materials are created, which are composed of amorphous arrays of submicrometer-sized fine spherical silica colloidal particles. When the colloidal amorphous arrays are thick, they do not appear colorful but almost white. However, the saturation of the structural color can be increased by (i) appropriately controlling the thickness of the array and (ii) placing the black background substrate. This is similar in the case of the blue feather of Steller's jay. Based on the knowledge gained through the biomimicry of structural colored materials, colloidal amorphous arrays on the surface of a black particle as the core particle are also prepared as colorful photonic pigments. Moreover, a structural color on-off system is successfully built by controlling the background brightness of the colloidal amorphous arrays.

Spontaneously Assembled Nano-aggregates in Clear Green Tea Infusions from Camellia ptilophylla and Camellia sinensis.

Tea nano-aggregates spontaneously assembled in clear tea infusions are considered as the precursors of tea cream, although their molecular basis remains obscure. Here, we characterized nano-aggregates in green tea infusions from Camellia ptilophylla, a peculiar tea variety with 6.0% of theobromine, and Camellia sinensis as the control for comparative purpose. Numerous negatively charged spherical colloidal particles of 50-100 nm in diameter were primarily found in both green tea infusions. Catechins, proteins, and carbohydrates were confirmed as the dominant components in green tea nano-aggregates. In addition, iron, copper, nickel, proteins, and gallated catechins exhibited higher aggregating affinity than other components, whereas methylxanthines and calcium contributed to the transformation of nano-aggregates into tea cream. Green tea nano-aggregates were partly destroyed by simulated gastrointestinal digestion, and removing theses peculiar particles dramatically attenuated the bioaccessibility of methylxanthines, theanine, and some catechin monomers in green tea infusions. This study enhanced our knowledge of molecular interactions in the formation of green tea cream and provided insight into physicochemical profiles, phytochemical nature, and functional effects of green tea nano-aggregates.

A simple algorithm for the calculation of an approximate electrophoretic mobility of a spherical colloidal particle based on the modified Poisson-Boltzmann equation

A simple algorithm is presented for the calculation of an approximate electrophoretic mobility of a spherical colloidal particle in an electrolyte solution. The obtained expressions are based on an approximate form of the modified Poisson-Boltzmann equation taking into account the ion size effects through Carnahan-Starling activity coefficients of electrolyte ions (J Chem Phys 51:635,31). Agreement with the exact numerical results by Lopez-Garcia et al. (J Colloid Interface Sci 458:273,27) is good for most practical cases.

Real-space imaging of translational and rotational dynamics of hard spheres from the fluid to the crystal.

Using real-space imaging of single particles, we investigate the interplay between translational and rotational motion of tracer particles in suspensions of colloidal particles over a wide range of volume fractions from dilute fluid to densely packed crystal. To this end, we introduce a new type of spherical colloidal tracer particles containing two differently labelled fluorescent cores. The tracer particles can be combined with host particles enclosing a single fluorescent core and chemical and physical properties identical to the tracers. This leads to a system of spherical colloidal particles, in which spatio-temporal trajectories of rotation and translation of individual particles can be recorded simultaneously with full 360° resolution of rotational dynamics. Our analysis shows that translation and rotation of colloidal particles are uncorrelated and decoupled for all volume fractions irrespective of the phase of the particle system.

Response of a doublet to a nearby dc electrode of uniform potential.

The electric-field-assisted directed assembly of spherical colloidal particles near an electrode has been studied for nearly three decades. Recently, focus has shifted to the electric-field-assisted assembly and propulsion of nonspherical (i.e., anisotropic) particles. This paper describes calculations and results for a doublet of asymmetric ζ potential and size responding to a dc electric field. The doublet experienced a net vertical force that depended on both the asymmetry in ζ potential and lobe size. In addition, the doublet experienced a net lateral force perpendicular to the applied electric field. The lateral force depended on the difference in ζ potential of the two lobes, the lobe size asymmetry, and also the angle of inclination of the doublet. The net force was used to predict an apparent lateral velocity, which was found to be perpendicular to the applied electric field. In addition, the particle experienced rotation from a net torque that depended on the lobe size asymmetry and also the angle of inclination of the doublet. The magnitude of the predicted velocity was of the same order of magnitude as has been observed for particles responding to ac electric fields in experiments. These results demonstrate that lobe ζ potential, lobe size, and orientation of a colloidal doublet can be tuned to achieve propulsion.

Reversed electrophoretic mobility of a spherical colloid in the Modified Poisson-Boltzmann approach

Abstract The electrophoretic mobility of a spherical colloid particle immersed in a binary electrolyte is studied by using the Primitive Model Electrophoresis formalism, in combination with the Modified Poisson-Boltzmann equilibrium theory. This approach differs from the classical descriptions of electrophoresis, based on the Poisson-Boltzmann equation for point ions, in that the Primitive Model Electrophoresis theory takes into account consistently the ionic correlations and excluded volume contributions due to the finite size of the ions. Using the equilibrium radial distribution functions of the Modified Poisson-Boltzmann, the primitive model electrophoresis theory predicts a non-universal behavior of the reduced mobility, as a function of the reduced ζ -potential, in contrast to the universal behavior exhibited by the well-known treatment of Wiersema, O’Brien and White. The primitive model electrophoresis mobilities calculated with the Modified Poisson-Boltzmann theory compare favorably with the previously reported mobilities obtained through the hypernetted-chain/mean spherical approximation equilibrium theory. The agreement between these two theories is very good, both quantitatively and qualitatively for univalent electrolytes. For 2:2 electrolytes they qualitatively agree. In particular, the Modified Poisson-Boltzmann mobilities confirm the occurrence of reversed electrophoretic mobilities. Overall the results presented here show the adequacy of the Modified Poisson-Boltzmann equilibrium theory to calculate the electrophoretic mobility of colloidal particles.

Morphology of Colloidal Particles Dispersed in Nematic Solvent

We have studied a system of spherical colloidal particles suspended in nematic liquid crystal confined to a two-dimensional plane. The dispersed colloidal particles pervert the uniform orientation of nematic resulting in topological defects. This small change in director field induces elastic interaction in the system. Considering the system exhibiting octopolar symmetry, the interaction of the particles can be described by octopole-octopole interaction potential which on some suitable scaling has the form, βu(r) ≈ Γ/r7, where Γ is dimensionless interaction strength parameter. We have calculated the pair correlation function and radial distribution function of the system by employing Roger-Young's integral equation theory, where the mixing parameter a, is obtained by demanding the consistency in pressure via virial and compressibility routs. With the increase in interaction strength, the system is found to become more ordered.

Polydisperse spherical colloidal silica particles: Preparation and application**Project supported by the National Major Scientific and Technological Special Project during the Twelfth Five-year Plan Period of China (Grant No. 2009ZX02030-1), the National Natural Science Foundation of China (Grant No. 51205387), the Science and Technology Commission of Shanghai, China (Grant No. 11nm0500300), and the Science and Technology Commission of Shanghai, China (Grant

A new industrial method has been developed to produce polydisperse spherical colloidal silica particles with a very broad particle size, ranging from 20–95 nm. The process uses a reactor in which the original seed solution is heated to 100°C, and then active silicic acid and the seed solution are titrated to the reactor continuously with a constant rate. The original seeds and the titrated seeds in the reactor will go through different particle growth cycles to form different particle sizes. Both the particles’ size distribution and morphology have been characterized by dynamic light scattering (DLS) and the focus ion beam (FIB) system. In addition, the as-prepared polydisperse colloidal silica particle in the application of sapphire wafer’s chemical mechanical polishing (CMP) process has been tested. The material removal rate (MRR) of this kind of abrasive has been tested and verified to be much faster than traditional monodisperse silica particles. Finally, the mechanism of sapphire CMP process by this kind of polydisperse silica particles has been investigated to explore the reasons for the high polishing rate.

Kinetically guided colloidal structure formation

The self-organization of colloidal particles is a promising approach to create novel structures and materials, with applications spanning from smart materials to optoelectronics to quantum computation. However, designing and producing mesoscale-sized structures remains a major challenge because at length scales of 10-100 μm equilibration times already become prohibitively long. Here, we extend the principle of rapid diffusion-limited cluster aggregation (DLCA) to a multicomponent system of spherical colloidal particles to enable the rational design and production of finite-sized anisotropic structures on the mesoscale. In stark contrast to equilibrium self-assembly techniques, kinetic traps are not avoided but exploited to control and guide mesoscopic structure formation. To this end the affinities, size, and stoichiometry of up to five different types of DNA-coated microspheres are adjusted to kinetically control a higher-order hierarchical aggregation process in time. We show that the aggregation process can be fully rationalized by considering an extended analytical DLCA model, allowing us to produce mesoscopic structures of up to 26 µm in diameter. This scale-free approach can easily be extended to any multicomponent system that allows for multiple orthogonal interactions, thus yielding a high potential of facilitating novel materials with tailored plasmonic excitation bands, scattering, biochemical, or mechanical behavior.

Minimizers of the Landau–de Gennes Energy Around a Spherical Colloid Particle

We consider energy minimizing configurations of a nematic liquid crystal around a spherical colloid particle, in the context of the Landau-de Gennes model. The nematic is assumed to occupy the exterior of a ball of radius r_0, satisfy homeotropic weak anchoring at the surface of the colloid, and approach a uniform uniaxial state at infinity. We study the minimizers in two different limiting regimes: for balls which are small compared to the characteristic length scale r_0<<L, and for large balls, r_0>>L. The relationship between the radius and the anchoring strength W is also relevant. For small balls we obtain a limiting quadrupolar configuration, with a "Saturn ring" defect for relatively strong anchoring, corresponding to an exchange of eigenvalues of the Q-tensor. In the limit of very large balls we obtain an axisymmetric minimizer of the Oseen-Frank energy, and a dipole configuration with exactly one point defect is obtained.

Simultaneous characterization of rotational and translational diffusion of optically anisotropic particles by optical microscopy

We probe the roto-translational Brownian motion of optically anisotropic particles suspended in water with a simple and straightforward optical microscopy experiment that does not require positional or rotational particle tracking. We acquire a movie of the suspension placed between two polarizing elements and we extract the translational diffusion coefficient DT and the rotational diffusion coefficient DR from the analysis of the temporal correlation properties of the spatial Fourier modes of the intensity fluctuations in the movie. Our method is successfully tested with a dilute suspension of birefringent spherical colloidal particles obtained by polymerizing an emulsion of droplets of liquid crystal in a nematic phase, whose roto-translational dynamics is found to be well described by theory. The simplicity of our approach makes our method a viable alternative to particle tracking and depolarized dynamic light scattering.

Dynamics of Self-Propelled Janus Particles in Viscoelastic Fluids.

We experimentally investigate active motion of spherical Janus colloidal particles in a viscoelastic fluid. Self-propulsion is achieved by a local concentration gradient of a critical polymer mixture which is imposed by laser illumination. Even in the regime where the fluid's viscosity is independent of the deformation rate induced by the particle, we find a remarkable increase of up to 2 orders of magnitude of the rotational diffusion with increasing particle velocity, which can be phenomenologically described by an effective rotational diffusion coefficient dependent on the Weissenberg number. We show that this effect gives rise to a highly anisotropic response of microswimmers in viscoelastic media to external forces, depending on its orientation.

Heat-Resistant Crack-Free Superhydrophobic Polydivinylbenzene Colloidal Films.

Highly cross-linked poly(divinylbenzene) (PDVB) spherical colloidal particles with nano-, submicron-, and micron-sizes of 157.2 nm, 602.1 nm, and 5.1 μm were synthesized through emulsion and dispersion polymerization methods. The influences of particle size on the surface morphology, roughness, superhydrophobicity, and critical cracking thickness of colloidal films were studied in detail. The results show that PDVB colloidal films possess large water contact angle (CA) over 151°, belonging to superhydrophobic materials. Moreover, it is interesting to observe that the highly cross-linked network structure leads to PDVB film's excellent heat-resistance. The CA and rough surface morphology remain nearly unchanged after thermal-treatment of films at 150 °C for 24 h. In addition, no cracks were observed in films with thicknesses up to 8.1 μm, exceeding most of polymer and inorganic particle films reported in the literature. The simple and scalable preparation method, low-cost, superhydrophobicity, and excellent thermal stability endow the PDVB colloidal films with promising applications in advanced coating fields, especially when employed in the high-temperature service environment.

From local to hydrodynamic friction in Brownian motion: A multiparticle collision dynamics simulation study.

The friction and diffusion coefficients of rigid spherical colloidal particles dissolved in a fluid are determined from velocity and force autocorrelation functions by mesoscale hydrodynamic simulations. Colloids with both slip and no-slip boundary conditions are considered, which are embedded in fluids modeled by multiparticle collision dynamics with and without angular momentum conservation. For no-slip boundary conditions, hydrodynamics yields the well-known Stokes law, while for slip boundary conditions the lack of angular momentum conservation leads to a reduction of the hydrodynamic friction coefficient compared to the classical result. The colloid diffusion coefficient is determined by integration of the velocity autocorrelation function, where the numerical result at shorter times is combined with the theoretical hydrodynamic expression for longer times. The suitability of this approach is confirmed by simulations of sedimenting colloids. In general, we find only minor deviations from the Stokes-Einstein relation, which even disappear for larger colloids. Importantly, for colloids with slip boundary conditions, our simulation results contradict the frequently assumed additivity of local and hydrodynamic diffusion coefficients.

On a suspension of nearly spherical colloidal particles under large-amplitude oscillatory shear flow

The dynamics of a dilute, monodisperse suspension of nearly spherical particles that undergo Brownian rotations in an oscillatory simple shear flow is quantified, as a paradigm for large-amplitude oscillatory shear (LAOS) rheology of complex fluids. We focus on the ‘strongly nonlinear’ regime of LAOS, defined by ${\it\beta}\gg 1$ and ${\it\beta}/{\it\alpha}\gg 1$, where ${\it\beta}$ is a dimensionless shear rate (or Weissenberg number) and ${\it\alpha}$ is a dimensionless oscillation frequency (or Deborah number). We derive an asymptotic solution for the long-time periodic orientation probability density function of the particles. Our analysis reveals that the orientation dynamics consists of ‘core’ regions of rapid oscillation (on the time scale of the inverse of the shear-rate amplitude), separated by comparatively short ‘turning’ regions of slow evolution when the imposed flow vanishes. Uniformly valid approximations to the shear stress and normal stress differences (NSDs) of the suspension are then constructed: the non-Newtonian contribution to the shear stress, first NSD and second NSD, decays as ${\it\beta}^{-3/2}$, ${\it\beta}^{-1}$ and ${\it\beta}^{-1/2}$, respectively, at large ${\it\beta}$. These stress scalings originate from the orientation dynamics at the turning regions. Therefore, it is the occasions when the flow vanishes that dominate the rheology of this paradigmatic complex fluid under LAOS.

Amorphous and ordered states of concentrated hard spheres under oscillatory shear

Hard sphere colloidal particles are a basic model system to study phase transitions, self-assembly and out-equilibrium states. Experimentally it has been shown that oscillatory shearing of a monodisperse hard sphere glass, produces two different crystal orientations; a face centered cubic (FCC) crystal with the close packed direction parallel to shear at high strains and an FCC crystal with the close packed direction perpendicular to shear at low strains. Here, using Brownian dynamics simulations of hard sphere particles, we have examined high volume fraction shear-induced crystals under oscillatory shear as well their glass counterparts at the same volume fraction. While particle displacements under shear in the glass are almost isotropic, the sheared FCC crystal structures oriented parallel to shear, are anisotropic due to the cooperative motion of velocity–vorticity layers of particles sliding over each other. These sliding layers generally result in lower stresses and less overall particle displacements. Additionally, from the two crystal types, the perpendicular crystal exhibits less stresses and displacements at smaller strains, however at larger strains, the sliding layers of the parallel crystal are found to be more efficient in minimizing stresses and displacements, while the perpendicular crystal becomes unstable. The findings of this work suggest that the process of shear-induced ordering for a colloidal glass is facilitated by large out of cage displacements, which allow the system to explore the energy landscape and find the minima in energy, stresses and displacements by configuring particles into a crystal oriented parallel to shear.

Influence of Localized Surface Charge Distributions on the Dielectric Relaxation Spectra of Spherical Colloidal Particles in Aqueous Solution

The influence of localized charge distributions at the interfaces of spherical colloidal particles in aqueous suspension is discussed in the light of the effective medium approximation theory of heterogeneous systems. The approach is shown to give analytical results in the case of shelled particles with the presence of two distinct interfaces. The whole dielectric response of the system gives rise to different relaxation contributions, falling in different frequency regions and attributed to different polarization mechanisms, at a molecular level. In particular, the number and the strength of the dielectric relaxations associated with the presence of localized charges is discussed.